There is considerable interest in the development of transversal filters for use in the GHz range for radar and electronic signal processing applications. Non-optical prior art techniques, such as those involving the use of surface acoustic waves, are generally unsuitable for use in this frequency range. Fiber optic techniques have been developed which provide up to 15 taps in the 2-400 MHz range and such techniques are described, for example, in Chang, et al, "Fiber Optic Delay Line for RF Signal Processing", Electron. Lett. 13,678(1977) which discusses the theory of transversal filters; and in Taylor, "Novel Application of Fiber Optics", Plenum (1979) and Newton et al "Single Mode Fiber 1.times.N Directional Coupler", Opt. Lett. 8,60 (1983), which are discussed below. In addition, a fiber optic delay line filter of interest is disclosed in U.S. Pat. No. 4,128,759 (Hunt et al), which is also referred to below.
In general, two techniques using optical filters are of interest. In accordance with the first, optical delays have been obtained by employing a bundle of multimode fibers of fifteen different lengths wherein the end of a bundle is excited with an rf modulated optical signal to simultaneously excite the fibers making up the bundle. An injection laser or light emitting diode modulated by a driver receiving an rf input signal can be used to provide the optical input signal and the outputs of the fibers of the bundle are simultaneously imaged on a single photodetector. This approach is disclosed in the Taylor reference referred to above as well as in the Hunt et al patent. Among others, this approach suffers the disadvantage that because the ends of the fibers of the bundle are illuminated, a large fraction of the light falls on the fiber cladding or in the space between the fibers.
In the second approach, disclosed the Newton et al reference, a 1.times.N directional coupler, which includes an N-fiber output terminal array, is mated with a corresponding tap array on a continuous fiber coil comprising N loops of fiber to achieve coupling. In an example, a single 1.times.7 tapped delay line provides uniform coupling coefficients with 0.1-0.2 dB excess loss per tap. The disclosed device is a serial device proposed for use as transversal filter, and, although there is no discussion in the Newton et al reference of any extension of this approach to a large number of taps, a problem with such an extension of the basic approach is that, with a serial device, the excess tap losses (i.e., the losses in excess of the useful tap outputs) accumulate as the signal propagates down the line and thus the power at any one tap is affected by all tap losses at preceding taps. Thus, assuming a 0.1 dB loss per tap and 100 taps, the final tap would be 10 dB below the initial taps, thereby making it difficult to produce an array of equal strength signals.